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But whether that means a cub's on the way or Mei Xiang's just having a pseudopregnancy remains to be seen.

Zoo officials said Monday that Mei's slow rise of urinary progesterone levels could indicate she'll give birth to another longed-for cub -- or experience the end of a pseudopregnancy within 30 to 50 days.

Scientists have been tracking Mei Xiang's hormones since she was artificially inseminated April 26 and 27, the Smithsonian Conservation Biology Institute (SCBI) said. Scientists used frozen sperm from Hui Hui, a panda living in China, as well as fresh sperm from the National Zoo's Tian Tian.

Tian Tian has fathered all four of Mei's cubs to date: Tai Shan, who was born in 2005 and now lives in China; two-year-old Bao Bao, who will stay at the National Zoo for another two years, and two cubs who did not survive.

Giant panda fetuses do not start developing until the final weeks of gestation. A female panda can have a pseudopregnancy, when she demonstrates the behaviors and hormonal changes of pregnancy without actually being pregnant.

Typical signs of rising hormone levels include nest building, choosing to spend more time in her den, sleeping more and eating less.

The only way to confirm a panda is pregnant is with an ultrasound, but that can be difficult because panda fetuses are tiny. None of Mei Xiang's cubs were ever seen on an ultrasound. At this point in time, it would also be too early to detect a fetus.

You can watch Mei Xiang on the National Zoo's panda cams and you can get all the latest updates on Mei Xiang by searching #PandaStory on Instagram.